Comparison of different methods of nitrogen-vacancy layer formation in diamond for widefield quantum microscopy

TL;DR

This study compares three methods for creating nitrogen-vacancy layers in diamond for widefield magnetic microscopy, evaluating their sensitivities, strain effects, and practical considerations at room and cryogenic temperatures.

Contribution

It provides the first direct experimental comparison of NV layer fabrication methods, highlighting the effectiveness of low-energy irradiation of HPHT diamond as a cost-effective approach.

Findings

HPHT and δ-doped samples achieve similar magnetic sensitivities within a factor of 2.

N$^+$ and CN$^-$ implanted samples have 2-5 times worse sensitivity.

Low-energy irradiation of HPHT diamond is a competitive, simple, and low-cost method.

Abstract

Thin layers of near-surface nitrogen-vacancy (NV) defects in diamond substrates are the workhorse of NV-based widefield magnetic microscopy, which has applications in physics, geology and biology. Several methods exist to create such NV layers, which generally involve incorporating nitrogen atoms (N) and vacancies (V) into the diamond through growth and/or irradiation. While there have been detailed studies of individual methods, a direct side-by-side experimental comparison of the resulting magnetic sensitivities is still missing. Here we characterise, at room and cryogenic temperatures, $\approx100$ nm thick NV layers fabricated via three different methods: 1) low-energy carbon irradiation of N-rich high-pressure high-temperature (HPHT) diamond, 2) carbon irradiation of $\delta$-doped chemical vapour deposition (CVD) diamond, 3) low-energy N$^+$ or CN$^-$ implantation into N-free CVD diamond. Despite significant variability within each method, we find that the best HPHT samples yield similar magnetic sensitivities (within a factor 2 on average) to our $\delta$-doped samples, of $<2$~$\mu$T Hz$^{-1/2}$ for DC magnetic fields and $<100$~nT Hz$^{-1/2}$ for AC fields (for a $400$~nm~$\times~400$~nm pixel), while the N$^+$ and CN$^-$ implanted samples exhibit an inferior sensitivity by a factor 2-5, at both room and low temperature. We also examine the crystal lattice strain caused by the respective methods and discuss the implications this has for widefield NV imaging. The pros and cons of each method, and potential future improvements, are discussed. This study highlights that low-energy irradiation of HPHT diamond, despite its relative simplicity and low cost, is a competitive method to create thin NV layers for widefield magnetic imaging.